The two opposite ends of the blade design spectrum are the AF (airflow) and SP (static pressure) designed fans. AF style fans have many thin blades (9-15), typically with a relatively steep angle. This is a highly efficient design to move airflow (volume) with the least amount of fan blade noise. However, the effectiveness decreases at a measurable rate when faced with resistance, like a dust filter or radiator. This blade design also will draw in and push air out in a wide conical pattern. This can be a positive or a negative. Not great for directly cooling hardware (like a m.2 drive), but perfect for rear exhaust with a high volume, wide draw area.

SP or other blades with a static pressure emphasis go the other direction. There should be a fewer number of blades (usually 7), they are wide with little space between those blades, and their rake is typically rather flat. As such, it is always going to be less capable of moving a volume of air at the same speed as an AF-style fan. However, it is far less impacted by resistance like radiators or dust filters. It also has a more focused airflow cone making it better for directly cooling hardware or pushing air through that radiator.

Which one is better? It depends on the application and more often how much fan speed you are willing to tolerate. The AF fan will still move a ton of air at high speeds, but you will have to live with the noise. If you are trying for a super quiet set-up, the SP will fare better with dust filters and radiators at low speed.

The problem is most people need some of both qualities and that can be addressed by creating a blade design in between the two. I generally refer to this as a 'hybrid' design, but there is no official moniker. These fans move a good amount of air, don't wilt when faced with resistance, and can serve as both case fans and radiator fans without much compromise. All of the new RGB line (ML, HD, LL, and even the SP-RGB) are this hybrid type design and can be effectively used in any case position. There are some subtle differences (7 blades vs 9, slight angle changes) that account for statistical differences, but when running at the same speed you will have nearly identical performance in most applications.

As such, you should pick the one that is most visually appealing to you. There are some requirement differences. SP120-RGB is 3 pin DC motors. All others are 4 pin PWM, necessary for i-cooler control. LL fans must have Link running at all times to maintain their lighting. HD and ML do not. The only one with a major constructive difference is the ML with its very different mag lev bearing. It is the quietest at comparable speeds and where a HD140 might start to get noticeable around 850-900, the ML140 can stretch out a little further before it becomes attention grabbing. For people going for the lowest noise possible, that is something to consider. The most noticeable other difference is the quantity of LEDs. The ML and SP-RGB will have the traditional 4, although the hub design makes a much better spectrum of light than the old 4 corner spotlight. The HD have 12 and the LL 16. That means they are bright! You really want Link lighting control through a Commander Pro and Lighting Node Pro so you can take full advantage of the lighting. This is a requirement for LL fans. The HD have a little wired thumb remote, but I strongly suggest you get one of the above devices for software control. It is a dramatic difference in control level. Best thing to do is take a look at some youtube videos to get a better idea of what the lighting patterns look like.

c-attack put a lot of good things, I will add to it. This is a confusing and cool area of discussion.

Air flows is measuring the flow of the fan in an unobstructed area. The rule of thumb is use higher air flow for unobstructed areas like exhaust/intakes and use higher static pressure on radiators.

As mentioned prior, the blades can be optimized for air flow or static pressure. They both will generate airflow and static pressure to some extent. When it comes to static pressure RPM correlates directly to static pressure. The higher the RPM the static pressure will jump. This comes at the cost of noise.

Other Options: SP-120 RGB (you get less color controls and you only have 4 LEDs in the center hub)

The HD-120 are around ~25% cheaper than the LL-120, though you still need a lighting node pro, if you want to control the HD-120 via software, while the LL-120 comes with one. You do get 4x LED strips w/ magnets with the LNP, though you can also use the manual control or plug it into your motherboard with a little modding:

Even though I was able to get the HD-120 controlled via my motherboard, I ended up buying the Lighting Node Pro as I think iCUE, which replaces Link and CUE, is hands down the best RGB software out there. Just keep in mind the motherboard mod method requires that the motherboard has digital/addressable RGB strip support.